The present invention relates to a process for the chip-forming machining of thermally sprayed cylinder barrels.
For production of engines with thermally sprayed cylinder bearing surfaces, it is known from European document EP 1 141 438 B1 to cast, clean and degrease a cylinder crankcase, to roughen the cylinder bores by means of corundum or sand blasting, and then to thermally coat the cylinder bores. The coating and roughening operations use a template which covers cylinder bores which are not to be processed at that time and may have exchangeable consumable layers on the inner side. After the coating operation, the cylinder crankcase is machined to its final dimensions.
Despite the advantages of this process, the bonding of the sprayed layer which subsequently forms the cylinder bearing surface is in need of improvement. Furthermore, the outlay on equipment is very high. This is also true of the production costs.
One object of this invention is to develop a process which allows good bonding of the layer to be achieved, while production is as economical as possible, with as low a scrap rate as possible.
This object is achieved by a process for chip-forming machining of thermally sprayed cylinder bores of a cylinder crankcase in which a cylinder crankcase is cast, those surfaces of the subsequent cylinder bores which are to be thermally coated are roughened, the cylinder bores are coated by a thermal spraying process, and the cylinder bores are remachined to their final dimensions.
The following text describes a process for producing a cylinder crankcase with thermally sprayed cylinder liners, clearly revealing the advantages attributable to the invention. This route is selected since the advantages in some cases only manifest themselves at a completely different location. During production, a cylinder crankcase is cast. The material used for the cylinder crankcase, which is preferably produced as a shaped casting, particularly preferably as a pressure die casting, is a gray cast iron material or a light metal material, such as an aluminum alloy.
A cast cylinder crankcase according to the invention has an oversized dimension at least in the region of the cylinder head sealing surface and, in the case of a four-cylinder engine, has four cylinder bores arranged in line. The walls of the cylinder bores are provided with a bearing surface layer. The bearing surface layer is applied after preprocessing of the cylinder bores or of the cylinder crankcase, by way of a thermal spraying process and, in particular, a plasma and/or arc wire spraying process.
The materials used here are preferably thermal spraying materials which are customary for such purposes, preferably Fe-containing materials. The layer thickness of the bearing surface layer is usually several hundred micrometers and, preferably, at least 150 micrometers.
After casting, the cast cylinder crankcase is cleaned and degreased. Then, the surface of the cylinder bores is roughened. It is preferable for roughening to be carried out by way of a fluid which is blasted onto the walls at a high pressure (several hundred to several thousand bar). In this case, as in the case, for example, of sand blasting, it is also possible for solid particles, such as sand or corundum, to be added to the blasting fluid. In the present application, processes of this type are referred to merely as blasting processes or as blasting machining, for the sake of simplicity.
It is preferable to use a particle-free water jet to which 1-5% by volume of a liquid cleaning agent and/or a liquid preservative has been added. The use of the preservative at least reduces the risk of the blasted cylinder crankcase and the device suffering corrosion, with the cleaning agent being used for further or better removal of impurities or residual coverings, such as center sleeve parting agents used in pressure die casting.
If the blasting machining uses a water jet of this type, it is expedient that a cylinder crankcase which has been blasted in this manner does not require any complex cleaning processes. This is associated, inter alia, with a reduction in overall processing costs, less space being taken up by the machines and reduced investment costs.
The blast machining which is provided for the purpose of roughening can advantageously be used not only to roughen the surface to be coated, but also at the same time to clean and degrease the walls of the cylinder bores. In this case, it is particularly appropriate to use liquid cleaning agent. This, inter alia, reduces a working step and therefore the machine costs, with a simultaneous associated reduction in processing time and costs per item.
Since the hardness may vary along the axial extent of the cylinder bore, it is expedient for the time of action on the wall of the cylinder bore which is to be machined to be selected in such a manner that, where the hardness of the wall is lowest, the amount of material removed plus the internal diameter which then remains corresponds at most to the subsequent final dimension minus the minimum application of material required for the bearing surface layer. In this context, tests have shown that it is favorable to remove at most between 0.020 and 0.140 mm, preferably between 0.004 mm and 0.006 mm.
In anticipation of the further process, it should also be mentioned in this context in particular that to improve the subsequent bonding of the sprayed layer to the cylinder crankcase, it is expedient for the cylinder bore—as seen in the direction of the crankshaft—to also be roughened at least slightly (a few mm) beyond the region of the subsequent cylinder bearing surface on the crankshaft side in the manner described, and also for it to be cleaned, degreased and if appropriate machined in a suitable way.
Surprisingly, it has been established that when using a (high-pressure) blasting process, it is expedient for a lance which guides the jet and is directed onto the wall of the cylinder bore to be operated in such a manner that it is for some time activated outside the cylinder bore. This means that it is operated at most at the subsequent working pressure with the fluid, in particular with water or a fluid provided with solid particles. Then, the lance is introduced into the cylinder bore, and the walls are covered by the jet and processed in the desired way.
Of course, to reduce the operating times and therefore also, inter alia, to reduce costs, it is expedient for a plurality of roughening lances to be used for the roughening and/or the machining by means of blasting and/or the cleaning and degreasing which is expediently carried out in the process, in particular by the addition of the cleaning agents and/or preservatives. Surprisingly, it has been established that with this procedure it is expedient for these lances to be operated in such a manner that they are activated not only—as mentioned in the case of the use of a single lance—for a certain time outside the cylinder bore, but also for the jets which escape in the process to be directed onto a baffle surface. The baffle surface in this case advantageously serves to stabilize the working jet and therefore to improve the reproducibility of the working results.
In a preferred configuration of the baffle surface in the form of a hollow cylinder, in which the jet outlet for the jet is to be arranged, the lances can, in an improved version, be rotated without problems, as is already the case for covering the walls of the cylinder bores with the jet. This reduces interference with the two lances by the other lance in each instance. The baffle surfaces which concentrically surround the lances are preferably made from a hard metal or reinforced steel (e.g. in particular case-hardened steel No. 1.7131 (16MoCr5)), so that the abrasive action of the jets, which are passed out of the lances from a fan jet nozzle, despite their extremely high pressure, is relatively slight, and the service life of baffle surfaces of this type is very long.
The same objective is also served by the use of a single pressure source for the lances which are in use, since in this case identical or similar working results can be assumed for the cylinder bores which have been processed in each instance. This has a particular influence on the abovementioned machining of the wall of the cylinder bore which is subsequently to be coated.
If a plurality of lances are used for simultaneous processing of a plurality of cylinder bores belonging to a line of cylinders, these lances are expediently not introduced simultaneously into cylinder bores which directly follow one another. Instead, at least one cylinder is to be left clear between the cylinder bores into which the lances are introduced. This, inter alia, reduces the influence which the lances have on one another and the potential threat to the lances, in particular from their jets. In particular, the guidance of the lances is also simplified, since the free space between two lances is increased.
In cylinder crankcases with an odd number of cylinder bores, when using a plurality of jet lances simultaneously, it is expedient for at least one to be configured such that it can be moved in the axial direction independently of the other and/or for them to be operated separately from one another with the blasting fluid, i.e. in particular with water. As a result, by way of example, one of the lances, when it is not arranged in a cylinder bore, can be switched off and/or operated in the region of the baffle surface and/or remain in an at-rest position, while the other blasting lance is applying the jet to one of the cylinder bores.
It is preferable for the cylinder crankcase to be fitted with one or more hollow-cylindrical baffle surfaces within the blasting machining unit, specifically in the region in which the blasting lances are also introduced into the cylinder bores. It is expedient for the baffle surfaces in this case to be arranged on a guide, by means of which they can be lowered onto the cylinder head sealing surface and put or placed and fixed on it in a defined position. For this purpose, the guide for the baffle surfaces is expediently arranged around a lance, so that the lance can be arranged within the hollow-cylindrical baffle surface and the baffle surface can be guided concentrically around the lance.
After blast machining, the cylinder crankcase, which may have been preprocessed, in particular face-milled, on the cylinder head sealing surface but still has an oversized dimension of preferably between 0.3 and 0.7 mm and particularly preferably between 0.4 and 0.5 mm, is removed from the blast machining unit. The blast machining unit preferably has a turntable with at least two receptacles, so that simultaneous loading of the machining unit in the region of the lances and removal and/or loading of a receptacle is possible. Alternatively, this can also be realized by linear drives or other similar customary production tools.
After blast machining, i.e. at least roughening the walls of the cylinder bores, the cylinder crankcase is tilted. For this purpose, the cylinder crankcase is rotated about a longitudinal axis of a cylinder line, so that an acute angle between the axial axis of a cylinder bore and the field line of the weight is present at least in a rotary position which is held at least for a short time. It is preferable for the cylinder crankcase, starting from the initial position defined by the cylinder head sealing surface facing upwards, is rotated through more than 90° (120°, 170°). In this context, it is proven expedient for the cylinder crankcase to be rotated as slowly as possible. In particular, the movement into this tilted position should last at least five seconds. It is preferable for at most the time which can be made available by the cycle time of the processing line to be used for this tilting. It is expedient for the cylinder crankcase even to be rotated through one full revolution about its longitudinal axis. During this tilting, inter alia the blasting fluid is at least in part removed in a simple way from the cylinder bores and in particular from desired recesses or undercuts and also recesses and/or undercuts which are also desired for the bonding of the subsequent coating and which were formed by the roughening operation.
It is expedient for the tilting of the cylinder crankcase to take place during the time in which it is being transported from the point of removal from the blasting machining unit into a drying processing unit. During this time, the cylinder crankcase can expediently also be acted on by compressed air which has preferably been heated to at least 50° C., particularly preferably to at least 70° C. This measure improves the removal of the blasting fluid still further.
In the drying processing unit, the cylinder crankcase is heated after it has been roughened, and the residual moisture is at least substantially, and optimally completely, removed. A standard furnace, the interior air of which is continuously recirculated and if appropriate also dried, can be used for this purpose. Furthermore, within the furnace it may be expedient to continue to blow hot air onto the cylinder crankcase.
After drying, the cylinder crankcase is transferred to a thermal coating processing unit, inside which the cylinder bores are coated in order to form what will subsequently be the cylinder bearing surfaces. The thermal coating processing unit, like the blasting processing unit, preferably likewise has a turntable with at least two receptacles, so that in this case too simultaneous loading of this processing unit in the region of coating lances and removal and/or loading of a receptacle is possible. Alternatively, this can also be realized by linear drives or other standard production tools of this type.
It is preferable for the cylinder crankcase to be provided with at least one spraying template, which is designed as a piece of tube and-therefore in hollow-cylindrical form and the clear width of which is greater than the clear width of a cylinder bore, in the region of the coating processing unit. The axial length of this spraying template approximately corresponds to the width of the thermal spraying jet, i.e. approx. 20 to 30 mm. The spraying template is preferably fitted outside the region of the coating processing unit in which a coating lance is arranged. As a result, in the case of a spraying template which has already been used, this template can be checked and if necessary removed in good time, for example if it is excessively soiled and/or the coating material is thought to have low adhesion, without involving significant intervention in the actual production sequence. It is expedient for the spraying template to be put or placed in a defined position on the cylinder head sealing surface in the region of the outer receptacle of the coating unit and fixed there. In this case, a single tubular spraying template is provided for a cylinder bore; it is possible to provide at least a single cylinder bore, but also a plurality of or all of the cylinder bores, of a cylinder crankcase with an associated tubular, hollow-cylindrical spraying template. In this case, a preferably continuous circular ring is formed between the inner wall of the spraying template and the cylinder crankcase in the region of the cylinder bore shielded by the spraying template. The thickness of the circular ring is at most 1 cm and is preferably between 0.3 and 0.7, particularly preferably approximately 0.5 mm.
The cylinder crankcase, which has now preferably been provided with a hollow-cylindrical spraying template similar to a piece of tube on the cylinder head sealing surface and still has an oversized dimension of preferably between 0.3 and 0.7 mm, particularly preferably between 0.4 and 0.5 mm, is introduced into an inner region of the coating processing unit, in which the coating of the walls of the cylinder bores is to be carried out by a thermal spraying process. The cylinder crankcase, which is held at a defined position with a spraying template arranged in a defined position on it, is transported under a coating lance, which is configured such that it can rotate about its longitudinal axis.
Furthermore, if only a single hollow-cylindrical spraying template is being used, the cylinder crankcase may likewise be configured such that it can rotate about this axis, the axis then being aligned with the axis of the individual cylinder bore which is to be coated. When adopting a procedure of this nature, it is advantageous if in each case only a single cylinder bore of a line of cylinders is thermally coated.
For this purpose—as also in the other cases—it is possible for an individual hollow-cylindrical, tube-like spraying template to be provided for each individual cylinder bore or for a plurality of cylinder bores or for all the cylinder bores; the spraying template may for its part once again be arranged on a base plate.
It is preferable for the coating lance to be started outside the cylinder bore, in order to rule out transient initial effects. In this case, the spray jet may, for example, be directed onto the inner surface of the spraying template, in order in particular to reduce soiling of the installation. After an initial running time, the coating lance is moved into the cylinder bore, and the coating is applied in a desired minimum thickness in accordance with a predeterminable working sequence. During the application of the coating, it is expedient for a gas, preferably an inert gas, to be passed through the cylinder bore so as to partially remove spray particles from the bore. The flow velocity in this case is expediently between 7 and 12, preferably approximately 10 m/s.
To coat a respective cylinder bore, a single cylinder bore or a plurality of cylinder bores or all the cylinder bores can be with and without separate retraction and refitting with spraying template.
The processing station described for the coating of the cylinder bores with a layer which has been thermally sprayed and preferably applied by means of an arc wire spraying process (AWS process), therefore includes a blasting processing unit and a coating processing unit. It is expedient for each of these units to be assigned a loading station, which simultaneously forms a removal station, so that the units can be loaded with cylinder crankcases for the respective working step and finished cylinder crankcases can be removed again at the same station. The blasting and coating processing units are also assigned a dry processing unit, in which the cylinder crankcase is heated after the blasting operation and the blasting fluid is at least substantially removed.
After the cylinder bore has been coated, the cylinder crankcase is removed from the coating processing unit and transferred to the further, chip-forming processing operation. Here, the sprayed C, i.e. the cylinder barrel, preferably cylinder bore, is rough-honed, a bevel is introduced in the region of the cylinder head sealing surface, the cylinder head sealing surface is milled to its final dimension, preferably by means of disposable cutting tool tips, and the cylinder barrels are finish-honed to their final dimension in one or more steps. Apart from the rough-honing and finish-honing, which at least have to follow one another in terms of time, these machining working steps can in principle be carried out in any desired order.
It is appropriate, however, for the bevel to be introduced after the rough-honing, since in this case the finished cylinder bearing surface is no longer soiled or is only slightly soiled. The same also applies to the milling of the cylinder head sealing surface to its final dimension. It is expedient for the bevel to be formed in such a way, in terms of its inclination and depth of introduction, that it has no sprayed material, but rather only cast material, at the subsequent transition between the bevel and the cylinder head sealing surface which has been machined to its final dimension. It is therefore expedient for the bevel to be formed after the rough-honing and before the finish-machining of the cylinder head sealing surface and of the cylinder barrel. On account of this sequence, the finished cylinder bearing surface is no longer soiled or is only slightly soiled, and furthermore the bonding of the sprayed material is improved, since it is no longer at risk, for example by being lifted off by means of a cutting edge, from the face-milling of the cylinder head sealing surface to its final dimension. With all the actions which have been mentioned and also those which are yet to be described below, it is also expedient for the cutting edges of the respective machining tools to engage in the material which is to be removed from the outside, i.e. approximately parallel to the surface orthogonal. This at least reduces or even altogether prevents detachment of a material as a result of the cutting edge engaging from below with the material subsequently being lifted off by the cutting edge.
It is preferable for a cone (countersinking drill, milling cutter or the like) mounted in universally jointed fashion to be used to introduce the insertion bevel, the cone having, at its introduction end side, a guide pin, the external effective diameter of which is selected in such a manner that in terms of the machining tolerances it corresponds at most to the smallest clear width of the coated cylinder bore.
The guide pin is designed as a honing head, the cutting edges of which are matched to the final dimension of the cylinder bearing surfaces. It is preferable for the honing head to have cutting edges which can move in the radial direction and which can be locked in the at-reset position and in the working position. This allows the introduction of the bevel and the honing to be carried out in a single operation within one processing station.
It is expedient for the guide pin to be placed against the cylinder bore, oriented at an inclination with respect to the longitudinal axis of the latter in the region of the subsequent insertion bevel, and thereby aligned. After floating and/or vibrating and/or shaking alignment, the guide pin is oriented axially parallel to, and preferably aligned with, the longitudinal axis of the cylinder bore. After or during the axial orientation of the insertion pin, the latter is simultaneously lowered at least part way into the cylinder bore.
As in the region of the cylinder head sealing surface, inter alia to improve the bonding of the sprayed material, it is expedient for the coated cylinder bore of the cylinder crankcase to be provided with an end bevel in the region of the crankshaft outlet side. The end bevel is designed in such a manner that there is no coating material, but rather only the cast or base material, at the transition from the end bevel to the crankshaft space. After the end bevel has been introduced, coating material which has been deposited during the thermal spraying is removed at least partially from the crankshaft-side part of the cylinder crankcase, i.e. from the crankshaft space. The coating material is removed by a jet, preferably a liquid jet mixed with solid particles and/or a water jet, which is operated with a pressure of between 300 and 1000 bar, preferably between 300 and 600 bar. Between 1 and 5% by volume of cleaning agent and/or preservative are added to the jet. Therefore, the same fluid which was already used during the roughening operation can be used as blasting fluid for cleaning the crankshaft space. However, the difference is that a significantly lower pressure is used in this case. The material which continues to adhere to the walls of the crankshaft space after this pressurized-jet cleaning can remain there since, as tests have proven, it does not become detached even under extremely high loads. In this case too, to avoid unnecessary soiling of the cylinder bearing surface, it is expedient for the cylinder bore only to be machined to its final dimension after the end bevel has been introduced. Furthermore, it is also expedient for the machining of the cylinder bore to its final dimension only to be carried out after the finish-machining of the crankshaft space.
In the same way as for introduction of the insertion bevel, a circular milling cutter can be used to introduce the end bevel. It is expedient for a guide pin, the external effective diameter of which is selected in such a manner that in terms of the machining tolerances it corresponds at most to the smallest clear width of the coated cylinder bore, is expediently arranged at the introduction-side end of this circular milling cutter. The further formation and expedient procedures can be inferred from what has already been described above.
Alternatively, it is possible for a cone mounted in universally jointed fashion to be used to introduce the end bevel in the same way as for the introduction of the insertion bevel. It is expedient for a guide pin, the external effective diameter of which is selected in such a manner that in terms of the machining tolerances it corresponds at most to the smallest clear width of the coated cylinder bore, to be arranged at the introduction-side end of this cone. The further formation and advantageous procedures can be discerned from what has already been described.
As has already been mentioned, it is expedient to ensure that during chip-forming machining of the insertion bevel and/or of the end bevel and/or of the cylinder head sealing surface and/or of the crankshaft space and/or of the cylinder bore, the respective machining tool is guided in such a manner that the respective cutting edges penetrate into the layer material which is to be removed from the outside.
During the cleaning/removal of coating material from the crankshaft space, it is expedient for the cylinder to be covered. For this purpose, a ram is introduced into the cylinder or moved onto the end bevel. It is preferable for the ram to be placed against the end bevel in such a manner as to form a circumferential and/or end seal, and preferably for the ram to form a sealing closure at the end bevel with the aid of a seal.
In engines with cylinders arranged in a number of lines (for example V or W engines), to prevent the introduction of spray material it is expedient for the individual lines to be shielded from one another in the region of the crankshaft space at least during the thermal coating by the introduction of a shielding template. For this purpose, the shielding template preferably has an elastomer layer which is arranged between two metal sheets and which can be placed against the walls of the crankshaft space in the region between two rows. In this case, it is expedient for both the metal sheets and the elastomer layer to be of a shape which is the negative of the contour to be applied. It is preferable for the metal sheets, when the shielding template is put in place, to be at a distance from the wall, whereas the sealing elastomer of the elastomer layer bears against the wall.
Further expedient configurations of the invention are defined in the claims. Moreover, the invention is explained in more detail on the basis of exemplary embodiments illustrated in the figures.